WHY DO GUTTERS OVERFLOW?Gutters and downpipes can block with debris and overflow and regular cleaningor fitting a gutter leaf barrier is often the only solution. BUT MOST OVERFLOWS ARE NOT CAUSED BY BLOCKAGESAND THESE FACTS WILL HELP YOU UNDERSTAND WHY

AS/NZS3500.3 is referenced by the Building Code of Australia (BCA) for roof and storm water drainage standards and compliance.

Eaves gutters are fitted in a W pattern and downpipes are plumbed at the lowest points. The high point determines the gutter’s fall/slope and the roof area that each downpipe harvests.

A high point should be located at a roof valley.

Eaves gutters should have a minimum slope of 1:500, a fall of 2mm every metre.

The gutter's floor at the high point must be lower than gutter's lowest wall at the low point to allow excess water to drain over the high point to the neighbouring downpipe.

HOWEVER

Many new homes have downpipes fitted near the house corners but hipped rooves have a lesser roof area at the corners draining to each metre of guttering than the greater roof area furthest from the downpipes. Concentrated flow from the greater roof area then has the furthest to travel and is why most overflows during heavy rain occur between downpipes and paradoxically near the gutter high points.

Downpipes drain with a "weir flow" and this forms a vortex over the downpipe, creating an air core within the downpipe. For this reason and unless blocked, downpipes never contain less than two thirds air, even when the gutters are overflowing!

Many plumbers will recommend fitting larger downpipes to stop overflows but this is often unsuccessful if the water overflows before reaching the downpipe.

The BCA only requires eaves gutters to be designed for a 1:20 year Average Recurrence Interval (ARI). A region's ARI is actually determined by a five minute intensity rather than the region's ARI stated mm of rain per hour. 1:20 ARI's and brief but intense localised storms have become more frequent.

Box gutters are designed for a 1:100 ARI.

An upper roof gutter draining onto a lower roof commonly discharges its flow through a downpipe spreader but regulations do not require a downpipe to be fitted close to the area of guttering that receives the most concentrated flow. This regulatory oversight commonly results in overflows even though the plumbing may be technically compliant.

AS/NZS3500.3 (2003) states: "Eaves gutter systems, including downpipes, shall be designed and installed in accordance with clause 3.2 so that water will not flow back into the building". Despite this regulation, hundreds of thousands of houses have been fitted with high fronted gutters with no overflow provision, allowing water to flow back into the house.

Slotted gutters have a smaller effective carrying capacity than a same sized unslotted gutter. Many industry tradespersons fail to recognise the requirement for slotted gutters to harvest smaller roof areas as regulated for smaller standard gutters with the same effective carrying capacity.

Many new houses do not have downpipes at the front for aesthetic reasons to increase the street appeal of display homes.

Numerous building inspectors have a poor understanding of roof drainage compliance and the large numbers of new houses incorrectly certified each year as compliant is damning proof of this.

Many houses have retrofitted carports and pergolas that drain to the house guttering. In most instances, the combined roof area drained is not compliant with plumbing regulations and overflows are common.